For the past 25 years tamoxifen has been the mainstay for adjuvant treatment of postmenopausal patients with hormone-sensitive breast cancer. However, tamoxifen has some safety and tolerability issues, and its partial estrogen-receptor agonist activity may have efficacy implications. Highly specific aromatase inhibitors, of which anastrozole was the first, were introduced in the 1990s and have emerged as a potentially better tolerated and more effective class of agents targeting hormonally responsive breast cancer. This article provides a review of the clinical pharmacology of anastrozole (1 mg once daily) and reviews the first results of the ongoing Arimidex, Tamoxifen, Alone or in Combination early breast cancer trial, initiated in 1996. This randomized, double-blind multicenter trial compared tamoxifen (20 mg once daily) with anastrozole (1 mg once daily) alone and the combination of anastrozole plus tamoxifen, as adjuvant endocrine treatment for postmenopausal patients with operable, invasive, early breast cancer. The results of the Arimidex, Tamoxifen, Alone or in Combination trial show anastrozole to be an effective and well tolerated endocrine option for early breast cancer and provide evidence for its potential role in chemoprevention.